The present invention relates generally to multithreading (MT), and more specifically, to a core-oriented system control area (COSCA) for managing multiple threads in a computer.
Multithreading (MT) provides a means for increasing the number of processor threads that can operate in parallel within a single physical processor core without the need to add additional cores. Ideally, MT provides this increased capacity by having one or more threads use portions of the core hardware that are currently not being used by the other thread(s) running on the same core. For example, during the latency caused by a cache-miss or other delay in one thread, one or more other threads can utilize the core resources, thus increasing the utilization of the resources. Even though in practice, this sharing results in some interference between the threads and requires some additional hardware, MT still provides the ability to perform each thread's work using less hardware then would be required if each thread were to run on its own isolated core hardware. Often, additional benefit can be derived from MT when the sharing of hardware resources between threads also decreases the overall strain on the computer system to provide information, such as data from memory, to two unique cores.
Typically, although MT provides hardware savings, the addition of another work thread consumes the same coordination cost at the hypervisor level that would be required to provide increased capacity using an additional, separate core. In many instances, once a certain scaling ratio is achieved, the overhead to coordinate resources between work threads, whether run on a single or shared core, is substantial and can decrease or even outweigh the benefits seen by the ability to run an independent work thread. That is, in general there is more management overhead as the number of things to manage increases.